The present invention relates to a booster device for suppressing a reduction in an output voltage of a DC power supply when the output voltage is decreased.
For example, an electronic component installed into a vehicle is driven by an electric power being fed from a battery (DC power supply). In such battery, a battery voltage is decreased because a large electric current is output from the battery during the starter cranking. Further, when such a condition is combined together that an ambient temperature is low, in some cases the battery voltage is decreased to about 3 to 4 V from 12 to 14 V as the voltage in a normal condition. For this reason, the battery cannot feed the voltage that various electronic components installed into the vehicle need. Concretely, a lowest operating voltage of the electronic components such as a control circuit, semiconductor switches, and the like installed into the vehicle is normally about 6 V, so that in some cases the battery cannot normally operate these electronic components when the battery voltage is decreased to about 3 to 4 V.
Then, an explanation will be made in detail with reference to FIG. 6 hereunder. FIG. 6 is a circuit diagram showing a configuration of a load driving circuit in the related art. As shown in FIG. 6, a MOSFET (T101) is provided between a battery 106 (output voltage VB) and a load Rz, and a gate of the MOSFET (T101) is connected to a control IC 101. Normally, an NMOS is employed as the MOSFET(T101). Also, the control IC 101 includes a charge pump 102 for driving the NMOS, an overcurrent protection circuit 103 for protecting the MOSFET (T101) and wirings from an overcurrent, a control logic circuit 104, and a driver 105. Then, ON/OFF of the MOSFET (T101) is switched by the ON/OFF operation of a switch SW1.
Also, a positive-side output terminal (drain of the MOSFET) of the battery 106 is connected to a ground via a diode DA1, a resistor RA1, and a capacitor CA1. Then, a connection point between the resistor RA1 and the capacitor CA1 is connected to the control IC 101. That is, a stabilized voltage VBB is generated by a circuit that consists of the diode DA1, the resistor RA1, and the capacitor CA1, and this voltage VBB is used as a power supply voltage of the control IC 101. The voltage VBB is given by VBB=VB−Vf1, where Vf1 is a forward voltage drop of diode DA1. In this case, a voltage drop in the resistor RA1 is neglected because a resistance value of the resistor RA1 is about 3.9 Ω.
Here, in the condition that the output voltage VB of the battery 106 is decreased due to the starter cranking, or the like, a decrease of the voltage VBB can be prevented by the diode DA1 and the capacitor CA1 when this reduction time is a short time. However, when a decrease of the output voltage VB of the battery 106 extends over a long time that exceeds 1 second, the electric charges accumulated in the capacitor CA1 are consumed by the control IC 101, and thus the voltage VBB is decreased.
A constant voltage circuit, a constant current circuit, an amplifier circuit, etc. are contained in the control IC 101, and a large number of electronic components such as operational amplifiers, comparators, etc. are used. Therefore, a head room and a foot room of these components must be ensured, and a lowest operating voltage becomes about 6V in the normal control IC.
In contrast, the output voltage VB of the battery 106 is decreased to about 3 to 4 V at a time of starter cranking, as described above. Therefore, in order to operate normally the circuits in the control IC 101 by this voltage, the head room and the foot room of the operational amplifiers and the comparators must be made smaller, and thus the operational amplifiers and the comparators must be designed to have a special configuration. This situation yields an increase in cost of the IC. Also, when the voltage VBB is decreased, a boosting capability of the charge pump 102 is lowered. Therefore, the number of step-up stages of the charge pump 102 must be increased. This situation also results in an increase in cost.
[Patent Literature 1] JP-A-2006-5581
As described above, in the load driving circuit in the related art, when the output voltage of the DC power supply (e.g., the battery) is decreased for some reason (e.g., the starter cranking) and then falls below the lowest driving voltage that is applicable to drive the electronic component (e.g., the element provided in the control IC), in some cases such output voltage cannot cause respective electronic components to operate normally. As a result, the demand that a reduction in the output voltage of the DC power supply should be suppressed by all means is escalating.